The present invention concerns a cross-linked polyolefin foam which has been foamed with water and cross-linked by hydrolysing and condensing silyl groups on the polyolefin.
Cross-linked polyolefin foam is presently produced by using chemical foaming agents, such as azodiacarbonamide, which decompose on being heated and generate gaseous nitrogen. The cross-linking is usually achieved by the aid of a radical former, such as dicumylperoxide. The cross-linking reaction is also achieved with the aid of heat. Cross-linked polyethylene foam manufacturing processes have also been developed, but in their case cross-linking is accomplished with the aid of irradiation. The above-mentioned products have very low densities, for which reason no applications requiring strength and rigidity can be contemplated. When an organic peroxide is used as a cross-linking agent, control of the process is difficult because foaming and cross-linking process, are both temperature-dependent. Endeavours are, as a rule, to proceed so that when being foamed the polyethylene is partly cross-linked because in this way a more uniform foam is obtained. The above-mentioned processes require extra apparatus after the extruder for cross-linking and foaming.
Polyethylene is also cross-linked by grafting to it an unsaturated alkoxysilane and by hydrolysing and condensing silyl groups by treating the end product with hot water or steam, with the aid of a condensing catalyst. Nowadays, polyethylene that is cross-linkable in this way is available in the form of two mix components, one of them containing grafted alkoxysilane and the other containing condensation catalyst. These granulates are mixed prior to extrusion and the end product is cross-linked with hot water or steam in a separate process step. Also other silane cross-linking processes exist, which differ somewhat from the process just described. In one process, all raw materials are supplied directly into the extruder at the step in which the end product is being manufactured. A separate cross-linking step is required in this case too.
It is true that foam cross-linked with silane is being produced to some extent, but all these processes use azodicarbonamide as foaming agent, and cross-linking takes place by treating the end product with hot water or steam.